The present invention relates to an electric power steering apparatus having a ball screw mechanism.
There is known an electric power steering apparatus for driving an electric motor in accordance with a steering torque to assist steering by transmitting a rotational force of the electric motor to a rack shaft. Here, there is a case of using a ball screw mechanism for converting the rotational force of the electric motor into a thrust force of the rack shaft (refer to Patent Reference 1).
[Patent Reference 1]
Examined Japanese Patent Application Publication No. Hei6-504
Meanwhile, according to the electric power steering apparatus described in Patent Reference 1, a gear is integrally formed with an outer periphery of a nut of the ball screw mechanism and power of the electric motor is transmitted to the nut of the electric motor via the gear. Although when the nut and the screw shaft are rotated relative to each other, there is needed a circulating path for returning a ball from one end to other end of a rolling path, nothing is disclosed in Patent Reference 1 with regard to the circulating path. Therefore, it seems that a tube or a frame which is a general circulating path is used. However, when the gear is formed at the outer periphery of the nut, it is difficult to arrange the tube or the frame to ride over the gear. Therefore, it is to be conceived that the gear of the nut is arranged to displace from the rolling path to an outer side in an axis line direction according to the electric power steering apparatus disclosed in Patent Reference 1 regardless of drawings of Patent Reference 1.
However, when it is conceived that the gear of the nut is arranged to displace from the rolling path to the outer side in the axis line direction, a torque transmitted from the electric motor is inputted to one end of the nut and therefore, the nut is liable to be twisted and power cannot be transmitted smoothly. Further, although when a large torque is transmitted, a tooth width of a gear is to be increased in order to reduce a face pressure of a tooth face, thereby, a length of the nut in the axis line direction is increased to pose also a problem that compact formation of the electric power steering apparatus cannot be achieved.